Humans see Colour, Dogs do not

We the humans see colour, but dogs do not. Do you know why?
Human retina has two types o sensory cells : rods and cones. At twilight more than 100 million rods permit us to distinguish between the colourless bright light and the dark. During the day around six million cones help us see a variety of colour. The eyes of dogs possess very few cones, which are sensitive only to blue and yellow for this reason, dogs see the world differently from us. Even there are many people who are blind to red and green and cannot correctly distinguish between these two colour.